<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <ul id="list">
      <li>li1</li>
      <li>li2</li>
      <li>li3</li>
    </ul>
    <script>
      const ul = document.getElementById("list");

      const df = []; // <df></df>
      setTimeout(() => {
        for (let i = 0; i < 10; i++) {
          const li = document.createElement("li");
          li.innerHTML = "li-" + i;
          df.push(li);
        }
        // ul.append(df[0],df[1],...);
        ul.append(...df);
      }, 3000);
    </script>
  </body>
</html>
